In a recent semiconductor device, since circuit elements in a semiconductor chip are arranged with a high density and with a high level of functions, the amount of heat evolved at per unit volume of the chip tends to increase. This is also the case in the plastic package type semiconductor device which is relatively inexpensive. Therefore, a low thermal resistance package structure has been adopted in the plastic package type semiconductor device.
As typical types of the low thermal resistance package structure, a heat radiating type and a heat spreader type have been known. In these types, the semiconductor chip loaded on a heat radiation plate is molded with resin, the heat radiating type being one in which the heat radiation plate is exposed to the outside of the surface of the mold resin, the heat spreader type being one in which the heat radiation plate is sealed within the mold resin. Among their, the latter is recommended for the semiconductor device having an integrated circuit chip because of its high moisture-proof reliability.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional heat spreader type semiconductor device. As illustrated therein, a heat spreader 11 made of copper (Cu) is adhered to a lead 3 of a lead frame (hereinafter, the lead frame is referred to as LF) by means of an insulation tape 12 made of polyimide or the like. A semiconductor chip 1 is fixedly adhered to the heat spreader 11 by means of a die bonding material 4 made of silver (Ag) paste or the like. An electrode pad formed on the semiconductor chip 1 and the LF lead 3 are interconnected with a bonding wire 5. The entirety with the exeption of a part of the LF lead 3 is sealed with a sealing resin made of epoxy resin or the like.
This conventional semiconductor device is assembled as follows. A lead frame to which the heat spreader 11 is adhered with the insulation tape 12 is prepared, then Ag paste is applied on the heat spreader 11, and then the semiconductor chip 1 is loaded on the heat spreader 11 with the Ag paste being interposed. The Ag paste is cured by heat treatment at a temperature on the order of about 230 degrees Centigrade to fixedly adhere the semiconductor chip 1 to the heat spreader 11. Subsequently, the electrode pad of the semiconductor chip 1 and the LF lead 3 are interconnected by means of the bonding wire 5 made of gold (Au) according to a thermo-sonic bonding process (TS process). At this time, the lead frame and the semiconductor chip 1 are heated to the order of 230 degrees Centigrade. Next, resin sealing is conducted by utilizing a transfer molding process. Finally, the lead frame is plated, cut and formed to obtain a finished article.
In the foregoing heat spreader type semiconductor device, since it is assembled in such a manner that the heat spreader is previously adhered to the lead frame, the adhesive agent (insulation tape) is exposed to a considerable high temperature in both the die bonding and wire bonding processes, which results in the thermal hysteresis of the adhesive agent causing its deterioration.
On the other hand, the semiconductor chip has been more and more refined and integrated through refinement of the recent design rules while at the same time the area of the electrode pad is also being reduced. However, since the lead frame is not reduced in size enough to a degree corresponding to the refined degree of the semi-conductor chip, the distance between the semiconductor chip and a bonding point of the lead frame is made longer and a long wire bonding becomes necessary. In order to carry out the long wire bonding with excellent positional accuracy, supersonic energy must be made low, and, if the area of the electrode pad on the chip is reduced as described above, it becomes also necessary to miniaturize the Au ball for wire bonding.
Under such circumstances, in order to secure the adhesion strength of the bonding wire, it becomes necessary to maintain the semiconductor chip and the lead frame at a higher temperature (for example, 300 degrees Centigrade) when the wire bonding is carried out. Further, a stronger and less resistant connection of the chip to the island (in the conventional case as shown in FIG. 1, heat spreader) has also been called for as to the die bonding of the semiconductor chip. In order to meet this requirement, it is desirable to use Ag glass paste, but if such is is used, then a heat treatment at high temperature (for example, 350 degrees Centigrade) becomes necessary.
However, in the conventional heat spreader type semiconductor device, since deterioration of the adhesive agent (insulation tape) is of concern, such a high temperature heat treatment cannot be applied, and it is difficult to realize a highly adhesive and less resistant interconnection.
Further, in the conventional heat spreader-type semiconductor device, since it is impossible to practice TAB (tape automated bonding) system, which calls for a higher-temperature (up to 500 degrees Centigrade), the heat spreader type could not be applied to the semiconductor device using a TAB lead.